© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1558 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Adv. Mater. 2012, 24, 1558–1565 Auke J. Kronemeijer,* Enrico Gili, Munazza Shahid, Jonathan Rivnay, Alberto Salleo, Martin Heeney,* and Henning Sirringhaus* A Selenophene-Based Low-Bandgap Donor–Acceptor Polymer Leading to Fast Ambipolar Logic Dr. A. J. Kronemeijer, Dr. E. Gili, Prof. H. Sirringhaus Cavendish Laboratory University of Cambridge J J Thomson Avenue, Cambridge, CB3 0HE, UK E-mail: ajk76@cam.ac.uk; hs220@cam.ac.uk Dr. M. Shahid, Dr. M. Heeney Department of Chemistry and Centre for Plastic Electronics Imperial College, London, SW7 2AZ, UK E-mail: m.heeney@imperial.ac.uk J. Rivnay, Prof. A. Salleo Department of Materials Science and Engineering Stanford University 476 Lomita Mall, 239 McCullough Building Stanford, CA 94305, USA DOI: 10.1002/adma.201104522 Solution-processed field-effect transistors based on conjugated polymers have been widely investigated in order to realise low cost and flexible electronics. [1] The performance of polymer cir- cuits is, amongst others, determined by the mobility of charge carriers in the channel of the constituent transistors and para- sitic capacitances due to the spatial geometry of the circuits. [2,3] With carrier mobilities in unipolar polymer transistors recently reaching ∼1-2 cm 2 /Vs for both holes and electrons, [4–11] solution-processed complementary logic based on organic field- effect transistors (C-FET) exhibiting practical performances can be potentially realised. [11–16] However, compatibility between the semiconductors, the dielectric and the electrical contacts must be ensured for specific material combinations with every mate- rial working optimally under common processing conditions. Furthermore, C-FET fabrication requires patterning of the dis- tinct polymer semiconductors. To substantially simplify the fab- rication process, solution-processed ambipolar semiconductors, capable of conducting both holes and electrons, are of interest since only a single unpatterned semiconductor has to be depos- ited. Organic CMOS-like logic based on ambipolar transport has been demonstrated by the fabrication of inverters and ring oscillators, [17–19] and printed ring oscillators based on a conju- gated polymer were fabricated exhibiting oscillation frequen- cies of up to 12 kHz. [20] Ambipolar transport in a single polymeric semiconductor has been realised by carefully tuning the energy levels of materials such that carriers can be injected in both the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molec- ular Orbital (LUMO) of the semiconductor and by selecting appropriate gate dielectrics to prevent electron trapping. [1] Low bandgap donor-acceptor polymers based on diketopyrrolopyr- role (DPP) have emerged as a promising class of materials exhibiting high and balanced hole and electron mobilities. [21–26] The DPP acceptor core constitutes a planar moiety capable of forming π-π stacks in the solid state leading to efficient charge transport. Using a DPP-based polymer, [22] CMOS-like ring oscil- lators with record oscillation frequencies of up to 42 kHz were demonstrated. [27] The DPP acceptor core is for synthetic reasons often flanked by two thiophene rings. One interesting recent example is the alternating copolymer formed by polymerisation of this core with a second electron accepting benzothiadiazole (BT) monomer. The resulting polymer, PDPPBT, exhibits balanced electron and hole mobilities of ∼0.4 cm 2 /Vs. [24,25] While thi- ophene-based polymers exhibit great potential, a number of studies have shown evidence of enhanced charge transport properties by substitution of the sulphur atoms in the thiophene rings with selenium atoms, leading to selenophene-based poly- mers. [8,28–30] Significant for ambipolar transport is the fact that incorporation of the selenium atoms lowers the LUMO level of the polymers, [31] leading to less trapping of electrons and facili- tating better electron injection from the contacts. In an attempt to improve the performance of the PDPPBT copolymer, we have synthesised and characterised the selen- ophene derivative of PDPPBT, abbreviated PSeDPPBT, in which selenophene rings flank the DPP unit (Scheme 1). We com- pared the performance of the selenophene polymer PSeDPPBT directly to that of the thiophene polymer PDPPBT using bottom- contact top-gate transistors. We found that selenium substitu- tion led to increased mobilities, with saturation hole and elec- tron mobilities of 0.46 and 0.84 cm 2 /Vs, respectively. Because of the ease of processing and the advantageous carrier charac- teristics of PSeDPPBT, we subsequently fabricated CMOS-like inverters and ring oscillators using a self-aligned gate tech- nique combined with a downscaled gate dielectric. [3] Using this approach, field-effect transistors with low gate leakage and small parasitic gate overlap capacitance were integrated to fab- ricate logic inverter gates and ring oscillators. Inverters exhib- ited high gains of ∼40, while three-stage ring oscillators showed stable output oscillation at supply voltages as low as 10 V. A maximum oscillation frequency of 182 kHz was determined at a driving voltage (V DD ) of 50 V. The combined progress in mate- rial performance together with the self-aligned gate technique enabled the ring oscillator to yield a stable output at compara- tively low supply voltages as well as fast oscillation at higher supply voltages. The polymers were synthesised by a Suzuki polycondensa- tion reaction of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan- 2-yl) benzothiadiazole with the respective thiophene [5] and